The ability to carry out photochemical reactions selectively and in a spatiotemporally controlled manner plays a key role in various modern technologies, including microfluidic devices (Duffy et al., 1998, Anal. Chem. 70 (23):4974-4984), metamaterials (Burckel et al., 2010, Adv. Mater. 22 (44):5053-5057), artificial tissues (Khademhosseini & Langer, 2007, Biomat. 28 (34):5087-5092), parallel protein synthesis techniques (Fodor et al., 1991, Science 251 (4995):767-773), three-dimensional prototyping (Neckers, 1992, Polym. Eng. Sci. 32 (20):1481-1489), optical device fabrication, and microchip fabrication (Ito, in “Microlithography—Molecular Imprinting,” Springer-Verlag Berlin, Berlin, 2005, Vol. 172, pp. 237-245).
Conventional photolithography begins with preparation of a photoactive thin film cast onto a substrate, typically using spin coating techniques, followed by application of heat to evaporate the solvent. A pattern is then transferred to the photoresist using actinic masked or focused laser light. The resulting image is developed by immersion in a solvent that selectively removes the undesired material. Photoresists are generally classified as being negative or positive tone resists (Ito, in “Microlithography—Molecular Imprinting,” Springer-Verlag Berlin, Berlin, 2005, Vol. 172, pp. 37-245). Positive tone resists are rendered soluble by irradiation, typically due to degradation or modification of the polymer polarity, while negative tone resists utilize polymerization or crosslinking reactions to render them insoluble after irradiation.
While nano-imprint lithography and a variety of other techniques have emerged for the fabrication of two dimensional (2D) structures, the top-down construction of three dimensional (3D) structures still presents significant challenges. Arbitrary 3D shapes incorporating overhanging and free standing features are difficult, or impossible, to fabricate using current methodologies. Importantly, such structures are important for the development of photonic crystals (McElhanon et al., 2002, J. Appl. Polym. Sci. 85 (7):1496-1502), elastic metamaterials (Lai et al., Nature Mater 10 (8):620-624), and other critical micro devices.
3D structures are fabricated by tracing out a 3D pattern using modern optical direct write lithographic techniques, or through a tedious cycle of patterning, development, planarization and alignment of multiple sequential layers, each of which possesses its own 2D pattern (Lin et al., 1998, Nature 394 (6690):251-253; Liu et al., 2008, Nature Materials 7 (1):31-37). While direct writing reduces the total number of steps, it still presents significant challenges. First, 3D construction of non-contiguous objects in a liquid negative tone photoresist is often problematic. Unlike 2D lithography, 3D features must be built up layer by layer in a bath of liquid monomer when using a negative tone resist (Zhou et al., 2002, Science 296 (5570):1106-1109). Accordingly, each underlying layer must support the next layer, or overhanging features will sediment from the densification associated with the polymerization. Sedimentation is slowed by utilizing higher viscosity photoresists; however, this approach complicates the later removal of the unreacted resist while aiming to preserve fragile features. Alternative ablative techniques such as focused ion beam lithography share similar structural limitations, and freeform fabrication techniques, such as laser sintering, are limited in resolution and restricted to rapid prototyping of macroscopic objects such as machine tooling and artificial bone constructs (Leong et al., 2003, Biomat. 24 (13):2363-2378). Positive tone photoresists in conjugation with two-photon processes sometimes overcome this challenge to write channels and other hollow interconnected structures where minimal material needs to be removed (Jhaveri et al., 2009, Chem. Mat. 21 (10):2003-2006).
Macromolecular networks incorporating crosslinks formed by Diels-Alder cycloaddition have been reported in the scientific literature. Chen et al. reported thermally reversible Diels-Alder cycloaddition of a monomer with four furan moieties on each molecule and a monomer with three maleimide moieties on each molecule to form a highly-crosslinked network (Science 2002, 295:1698). After casting of a dichloromethane solution of the monomers (with stoichiometric furan-maleimide ratio), the solvent was vacuum evaporated at room temperature and the material was then heated. Cross-linking of furan side chain polymers with bis-maleimide-containing small molecules has also been reported (Sanyal, 2010, Macromol. Chem. Phys. 211:1417-1425).
Further, use of Diels-Alder crosslinked films for nanoscale probe lithography and data storage has been reported (Gotsmann et al., 2006, Adv. Funct. Mater. 16(11):1499-1505). U.S. Patent Application Publication No. 2009/0100553 discloses a scanning probe-based lithography method in which patterning of a resist medium is produced by Atomic Force Microscope probe-surface contact. In this report, the resist may be an organic polymer in which polymer chains are connected to each other with Diels-Alder adducts.
Polymer networks incorporating pendant Diels-Alder adducts have been described in the literature. Kosif et al. reported formation of a cross-linked polymer with pendant furan protected maleimide groups via gelation of a furan protected maleimide containing methacrylate monomer with a polyethylene glycol methacrylate monomer (Kosif et al., 2010, Macromolecules 43:4140-4148). After gelation, the protected maleimide groups were reportedly activated to their reactive forms via a thermal cycloreversion step. The dried hydrogel was added to a solution containing a fluorescent dye or a thiolated biotin derivative.
There is a need in the art for novel light-activated polymerizable compositions, wherein reversible crosslinks may be converted into irreversible crosslinks using a fully controllable physical or chemical process. Such compositions may be useful for photolithographic and 2D/3D prototyping applications. The present invention fulfills this need.